RESEARCH ARTICLE Rapid Vegetation Succession and Coupled Permafrost 10.1029/2019JG005618 Dynamics in Arctic Thaw Ponds in the Siberian Key Points: • Thaw ponds formed by small‐scale Lowland Tundra thermokarst in the Siberian lowland Rúna Í. Magnússon1 , Juul Limpens1, Jacobus van Huissteden2, David Kleijn1, tundra display rapid formation and 3,4 1 5 1 colonization by aquatic vegetation Trofim C. Maximov , Ronny Rotbarth , Ute Sass‐Klaassen , and Monique M. P. D. Heijmans • Colonization by aquatic plant 1 species, in particular Sphagnum Plant Ecology and Nature Conservation Group, Wageningen University and Research, Wageningen, The Netherlands, mosses, is associated with recovery 2Earth and Climate Cluster, Department of Earth Sciences, Faculty of Earth and Life Sciences, Vrije Universiteit, of permafrost in these ponds Amsterdam, The Netherlands, 3Institute for Biological Problems of the Cryolithozone, Siberian Branch Russian Academy • Recruitment of dwarf shrubs on of Sciences, Yakutsk, Russia, 4North‐Eastern Federal University, Yakutsk, Russia, 5Forest Ecology and Management Sphagnum carpets in thaw ponds indicates potential recovery of the Group, Wageningen University and Research, Wageningen, The Netherlands original dwarf shrub‐dominated vegetation Abstract Thermokarst features, such as thaw ponds, are hotspots for methane emissions in warming Supporting Information: lowland tundra. Presently we lack quantitative knowledge on the formation rates of thaw ponds and • Supporting Information S1 subsequent vegetation succession, necessary to determine their net contribution to greenhouse gas emissions. This study sets out to identify development trajectories and formation rates of small‐scale (<100 m2), shallow arctic thaw ponds in north‐eastern Siberia. We selected 40 ponds of different age classes Correspondence to: ‐ R. Í. Magnússon, based on a time series of satellite images and measured vegetation composition, microtopography, water [email protected] table, and thaw depth in the field and measured age of colonizing shrubs in thaw ponds using dendrochronology. We found that young ponds are characterized by dead shrubs, while older ponds show Citation: rapid terrestrialization through colonization by sedges and Sphagnum moss. While dead shrubs and open Magnússon, R. Í., Limpens, J., water are associated with permafrost degradation (lower surface elevation, larger thaw depth), sites with van Huissteden, J., Kleijn, D., sedge and in particular Sphagnum display indications of permafrost recovery. Recruitment of Betula nana Maximov, T. C., Rotbarth, R., et al. ‐ (2020). Rapid vegetation succession and on Sphagnum carpets in ponds indicates a potential recovery toward shrub dominated vegetation, although coupled permafrost dynamics in Arctic it remains unclear if and on what timescale this occurs. Our results suggest that thaw ponds display thaw ponds in the Siberian lowland potentially cyclic vegetation succession associated with permafrost degradation and recovery. Pond tundra. Journal of Geophysical Research: Biogeosciences, 125, formation and initial colonization by sedges can occur on subdecadal timescales, suggesting rapid e2019JG005618. https://doi.org/ degradation and initial recovery of permafrost. The rates of formation and recovery of small‐scale, shallow 10.1029/2019JG005618 thaw ponds have implications for the greening/browning dynamics and carbon balance of this ecosystem. Received 17 DEC 2019 Plain Language Summary Global warming results in dramatic changes across Arctic Accepted 19 MAY 2020 landscapes, for instance the thawing of permafrost. Thawing of ice‐rich permafrost creates local depressions Accepted article online 8 JUN 2020 that may fill in with water, forming thaw ponds. In thaw ponds, previously frozen carbon becomes available for decomposition into the greenhouse gas methane. It is presently unknown how long thaw ponds remain sources of methane. We studied whether the permafrost and original dwarf shrub vegetation can recover after thaw pond formation. In the Siberian lowland tundra, we selected 40 shallow ponds of various ages, based on a series of aerial photographs. In these ponds, we assessed vegetation composition and the status of the permafrost. We found that once formed, thaw ponds are rapidly colonized by sedges. In older thaw ponds, carpets of peat moss appear, on which shrubs can reestablish. We found that the permafrost thaws less deeply under peat moss carpets than under open water or sedges. This indicates that thaw ponds go through a succession of dead shrub vegetation toward sedges and peat moss and potentially back to shrubs, and that this is related to the degradation and recovery of the permafrost. 1. Introduction ©2020. The Authors. This is an open access article under the The Arctic is warming twice as fast as the global average. It is expected that by the end of the 21st century, the terms of the Creative Commons global area of near‐surface permafrost will decrease by 24 ± 16% (IPCC scenario RCP2.6) to 69 ± 20% (IPCC Attribution License, which permits use, scenario RCP8.5) (Meredith et al., 2019). This thawing of permafrost exposes previously frozen soil organic distribution and reproduction in any medium, provided the original work is carbon to microbial decomposition, resulting in the release of greenhouse gases (GHG). This constitutes a properly cited. positive feedback to climate change (Schuur et al., 2015). The expected release of GHG from thawing MAGNÚSSON ET AL. 1of20 Journal of Geophysical Research: Biogeosciences 10.1029/2019JG005618 permafrost is in the order of 10s to 100s gt carbon by the end of the 21st century (Meredith et al., 2019). There is, however, considerable uncertainty about the exact magnitude of this “permafrost carbon feedback” (Meredith et al., 2019; Schuur et al., 2015). Part of this uncertainty revolves around the relative importance of two thawing mechanisms: gradual versus abrupt thaw. Gradually increasing thaw depths and higher temperatures have resulted in increased vegetation cover and productivity (Tape et al., 2006), most notably in shrubs (Myers‐ Smith, Elmendorf, et al., 2015), which may partly offset GHG emissions from thawing permafrost (Euskirchen et al., 2009). Satellite monitoring of the Normalized Difference Vegetation Index (NDVI), a mea- sure of vegetation greenness and photosynthetic capacity, confirms that the Arctic has shown a greening trend over most of the past decades (Epstein et al., 2018). In contrast, abrupt permafrost thaw occurs when excess ground‐ice in the permafrost melts, causing a collapse of the soil surface (thermokarst) (Schuur et al., 2015). Such processes may counteract the widely observed Arctic greening trend (Abbott et al., 2016; Phoenix & Bjerke, 2016; Raynolds & Walker, 2016). Since 2011, local browning (a decrease in NDVI) has diminished or even halted the overall Arctic greening trend (Epstein et al., 2018). Abrupt rather than gradual thaw may pre- dominate in large parts of the Arctic (Olefeldt et al., 2016; Schuur et al., 2015) and has been found to occur increasingly throughout Arctic regions (Farquharson et al., 2019; Fedorov et al., 2014; Frost et al., 2018; Jorgenson et al., 2006; Jorgenson & Grosse, 2016; Liljedahl et al., 2016; Raynolds et al., 2014; Walter et al., 2006). Abrupt thaw can lead to formation and expansion of surface water in continuous permafrost (Jorgenson et al., 2006; Raynolds et al., 2014) and is expected to increase the area of small lakes by 50% by 2,100 under IPCC scenario RCP8.5 (Meredith et al., 2019). It thereby enhances GHG emission through increased wet- ting and disturbance of the existing vegetation (Abbott et al., 2016). However, most studies on permafrost carbon dynamics have focused on gradual thaw, and abrupt thaw processes are currently not considered in climate models (Meredith et al., 2019; Schuur et al., 2015; Van Huissteden & Dolman, 2012). Thaw ponds are an example of features resulting from abrupt thaw of which we have limited understanding. Thaw ponds are isolated depressions of less than 1 ha which form due to local melting of ground‐ice. In these local depressions, snow and water accumulate, posing a positive feedback on thawing (Nauta et al., 2015). Thaw ponds are demonstrated sources of methane, as opposed to shrub‐dominated tundra vegetation (Nauta et al., 2015; Van Huissteden & Dolman, 2012; Van Huissteden et al., 2005, 2009). Thaw ponds are abundant throughout ice‐rich lowland tundra, but small waterbodies often go undetected in remote sensing analysis of thermokarst features (Campbell et al., 2018; Grosse et al., 2008; Raynolds et al., 2019). In contrast to the deep, ditch‐shaped ponds resulting from ice‐wedge degradation, melting of various other types of ground ice (e.g., ice lenses) may result in shallow, irregular shaped thaw ponds (Van Huissteden, 2020). Smaller, shallow ponds may be especially vulnerable to climate‐induced changes in hydrological regimes (Campbell et al., 2018; Wolfe et al., 2011). Such thaw ponds have received little scientific attention compared to other abrupt thaw features (e.g., thaw lakes, ice‐wedge degradation, retrogressive thaw slumps). This war- rants research into trends in shallow thaw pond area, the processes that trigger formation of such thaw ponds and their long‐term fate. Several studies have focused on long‐term development of thermokarst features related to ice‐wedge degra- dation (Jorgenson et al., 2015; Kanevskiy et al., 2017; Liljedahl et al., 2016) or thermokarst bogs in subarctic regions (Myers‐Smith